Method and apparatus for cleansing by ionic bombardment



1966 G. s. ANDERSON ETAL 3,233,137

METHOD AND APPARATUS FOR CLEANSING BY IONIC BOMBARDMENT 2 Sheets-Sheet 1Filed Aug. 28, 1961 FIG. I

Will/Illa,

,H MIHL l INVENTORS GERALD S. ANDERSON ROGER M. MOSEZON ATTORNEY UnitedStates Patent Office 3,233,137 Patented Feb. 1, 1966 3,233,137 METHODAND APPARATUS FOR CLEANSING BY IONIC BOMBARDMENT Gerald S. Anderson, St.Paul, and Roger M. Moseson, Minneapolis, Minn., assignors, by mesneassignments, to Litton Systems Inc., Beverly Hills, Califl, acorporation of Maryland Filed Aug. 28, 1961, Ser. No. 134,457 4 Claims.(Cl. 313-401) This invention relates to a method and apparatus forcleansing by ionic bombardment, and more particularly to such a methodand apparatus for cleansing nonconductive surfaces.

The technique of sputtering, which has been known for many years,involves the removal of atoms from a material as a result of bombardmentof the material by particles such as ions or atoms. It may be used foratomically cleaning the surface of a material, for etching the surface,or the sputtered material may be deposited as a coating on a nearbysurface if desired. Sputtering may also be used for other purposes suchas to provide samples of material for spectroscopic examination.

Sputtering has been accomplished in the past by directing a beam ofenergetic particles, either neutral or charged, at the target material.However, it is difficult to obtain beams with sufiicient intensity toremove atoms from the target material at a rate which is satisfactoryfor the deposition of the material on another surface to form a film orthe like.

Another method of sputtering, which is considerably faster than thatusing a beam of particles, involves immersing the target in a gasdischarge plasma, which may be produced by various methods such as aglow discharge, a low-pressure discharge with an electron source (suchas a mercury pool or a thermionic cathode) or by a highfrequencydischarge. The plasma in which the target is immersed is composed ofions, electrons and neutral atoms. When a potential that is negativewith respect to the plasma potential is placed in the target, thepotential difference appears across the Langmuir sheath which formsaround the target, and the target is bombarded by ions of a kineticenergy equal to this potential difference. Utilizing this technique, ametallic target can be sputtered very readily. However, it has not beenpossible to sputter nonconductors by this method because a positivecharge builds up on the target surface in a very short time and reducesthe potential difference between the target surface and the plasma tosuch a degree that it is too small for any appreciable sputtering.However, by connecting a source of alternating potential between theelectrode that holds the target and one of the other electrodes of theapparatus, a nonconductive material may be sputtered. This technique isdescribed in detail in copending application Serial No. 134,458 filedconcurrently.

In utilizing either the direct or alternating potential technique, thereis a tendency for the sputtered material to be deposited on the surfaceof the envelope within which the process takes place. Such envelopes areusually nonconductors and are often transparent so that the process maybe observed. Therefore, any deposition on the envelope which decreasesits transparency is undesirable. Heretofore, it has been necessary todismantle the apparatus in order to remove the deposit.

Such deposits tend to form not only in sputtering apparatus, but invirtually any gas discharge apparatus where sputtering occurs as asecondary or unwanted effect. For example, in the ordinary fluorescentlamp, dark deposits tend to form at each end of the glass envelope whichconsist of material sputtered from the lamps electrodes. Thus far, therehas been no practical way of removing such illumination-reducingdeposits.

Accordingly, a primary object of the present invention is to provide amethod and apparatus for preventing, or, if already formed, removingdeposits that tend to form on the enclosing envelope of gas dischargeapparatus.

Briefly speaking, the present invention utilizes ions existing in thegas discharge plasma of an apparatus or device to sputter off anundesired deposit from the envelope of the device. This is accomplishedby placing an electrode, preferably transparent, adjacent the envelopewhere the deposit is located or tends to form, and impressing analternating potential between that electrode and another electrode ofthe device. Thus, the deposit and the envelope are not only bombarded bypositive ions but are also bombarded by electrons for part of eachcycle, which prevents a residual or permanent positive charge frombuilding up on the surface of a nonconducting envelope. The deposit isquickly sputtered away.

Other objects and features of the invention will become apparent fromthe following description, taken in conjunction with the accompanyingdrawings, in which:

FIGURE 1 is a perspective view of a low-pressure mercury plasma tube towhich the present invention may be applied;

FIG. 2 is a vertical sectional view of the tube shown in FIG. 1;

FIG. 3 is a horizontal section view taken on the line 33 of FIG. 2;

FIGS. 4 and 5 are circuit diagrams for operating tubes according to theinvention; and

FIG. 6 is a fragmentary perspective view of a fluorescent lamp to whichthe invention is applied.

Although the present invention is illustrated and will be described inconjunction with a low-pressure mercury plasma tube, it is to beunderstood that it is not limited thereto. The teachings of theinvention may be applied with corresponding advantages and benefits toany apparatus or device which utilizes a gas discharge plasma in itsoperation. Furthermore, the invention is not limited to the use ofeither a direct current or an alternating current discharge device.

FIG. 1 illustrates a low-pressure mercury plasma tube 10 in conjunctionwith which the invention will be described. Such a tube is described indetail in a co-pending application of Gottfried K. Wehner, Serial No.103,- 056, filed April 14, 1961, now Patent No. 3,100,272, and entitledlow Pressure Mercury Plasma Discharge Tube. Therefore, it will bedescribed here in general terms only. The tube 10 has an upper chamberor envelope section 11 and a lower section or tank assembly 12 and isprovided with a large-diameter conduit 13 which connects the interior ofthe tank assembly 12 to a mercury diffusion pump (not shown). The tankassembly 12 is also provided with water inlet and outlet means 14 and15, respectively and with gas inlet means 16.

As best seen in FIG. 2, the tank assembly 12, which may be made of aconducting material such as stainless steel, includes three cylindricalwalls 17, 18, and 19, with the walls being spaced apart to formreservoirs 21 and 22 therebetween. The water inlet and outlet pipes 14and 15 extend through the outer cylindrical wall 17 and provide meansfor circulating water in the reservoir 21 to regulate the internaltemperature of the tube.

The reservoir 22 formed between the walls 18 and 19 contains a mercurypool 23, in the present embodiment of the invention. An igniterelectrode 24, making coutaet with the mercury pool 23 is connected to anelectrical lead 25.

In order to provide suitable means for mounting the envelope section 11,the Wall 18 is provided at its upper end with a shoulder portion 26. Theshoulder portion 26 has an annular groove 27 which contains an O-ring 3seal 23, and the envelope 11 is firmly but releasably positioned on theO-ring in the recess. Thus, a gas tight connection is provided betweenthe tank assembly 12 and the envelope 11.

The envelope 11 may be of conventional construction, and is customarilyof transparent construction such as a Pyrex glass bell jar. Also, it maybe provided with a reentrant cold trap cavity 30 extending downwardlyinto the envelope and so constructed that it can receive and retain acooling material such as liquid nitrogen.

The interior of the envelope 11 is separated from the interior of thetank assembly 12 by a circular plate 31 which is located in a seatformed in the shoulder portion 26 of the cylindrical wall 18. The plate31 is insulated from the shoulder by conventional insulating material,and has a mesh graphite grid electrode 32 positioned in a centralaperture formed therein. The grid is electrically connected to a lead33.

A plurality of electrodes are supported by the tank assembly 12, some ofwhich are located within the envelope portion 11 and some of which arelocated within the tank assembly 12. In effect, those electrodes locatedwithin the envelope 11 form the anode assembly, and those located withinthe tank 12 form the cathode assem- Located within the cathode space inthe base structure 12, along with the igniter electrode 24 previouslymentioned, is an auxiliary anode 34 which is electrically connected to alead 35.

Located within the anode space in the envelope portion 11 of the tubeare a main anode 36 which is electrically connected to a lead 37, and atarget electrode 38, which is adapted to hold a target material 40 to besputtered. The target electrode 38' is electrically connected to a lead41. The anode space may also contain an insulating support 42 for aplate or substrate on which material sputtered from the target 41) is tobe deposited.

The electrical leads from all of the electrodes thus far describedextend out of the base portion 12 of the tube through suitableconventional insulators. In addition, the leads 25, 35, 37 and 41 to theigniter 24, auxiliary anode 34, main anode 36 and target electrode 38may be of sufiicient strength to support the electrodes physically.

In the course of operation of the tube as thus far set forth, whichoperation will be later described, material sputtered from the targetvirtually fills the interior of the envelope 11 and tends to bedeposited on the envelope as well as on the substrate. This depositionoccurs at such a rate that after several hours of operation the envelopebecomes virtually opaque and the apparatus must be dismantled in orderto remove the deposit. In accordance with one of the teachings of theinvention, such deposition may be removed or a deposit prevented fromforming by placing an electrode 50 (having a lead 51) adjacent the outersurface of the envelope where it is desired to keep the envelopetransparent.

The structure of the electrode 50 is not critical, and it may take theform of a fine wire screen taped or otherwise secured to the envelope.Alternatively, it may be a transparent conductive paint of conventionaltype, or it could take the form of a conductive plate provided with asuitable handle so that it could be periodically held adjacent theenvelope.

Electrical connections to the apparatus for operation as a directcurrent discharge device are shown in FIG. 4, which also shows agrounded lead 53 that is connected to the base structure 12 of the tube.The lead 33 from the graphite grid electrode 32 is connected through avariable resistor 54 to the positive side of a conventional 30-voltdirect current (DC) power supply, the negative side of which isgrounded. The lead 25 from the igniter electrode 24 is connected to oneterminal of a two-way switch 55, and another terminal of the switch isconnected to the positive side of a conventional ZOO-volt DC. powersupply, the negative side of which is grounded.

4 The lever of the switch 55 is connected to ground through a capacitor56. The lead 35 from the auxiliary anode 34 is connected to the positiveside of the ZOO-volt power supply through a resistor 57, and the lead 37of the main anode 36 is similarly connected through a variable resistor58. The target electrode 38 is connected by means of its lead 41 througha resistor 59 to the negative side of a conventional 20-volt DC. powersupply, the other side of which is connected to the main anode.

In accordance with one important aspect of the inven tion, analternating electric field is applied between the external electrode 50and the discharge plasma existing within the tube during operation.Thus, as shown in FIG. 4, the electrode 50 is cdnneeted by means of itslead 51 through an inductance 60 to one side of a conventionalalternating current (A.C.) source, preferably of radio frequencies, theother side of which is connected through a DC. blocking capacitor 61 tothe main anode lead 37.

Although the alternating current source is shown in FIG. 4 as beingconnected between the external electrode and the main anode, it has beenfound in practice that it may be connected between the externalelectrode and any of the electrodes in the tube or between the externalelectrode and ground. Also, it may be connected between two electrodesarranged on opposite sides of the envelope. There is a small capacitanceexisting between the electrode 50 and'the gas discharge plasma, which isshown in broken lines at 62 in FIG. 4, and the purpose of the inductance60 is to form a series resonance circuit with that capacitance. Withoutthe inductance 60 in the circuit, considerably more power is requiredfor cleansing the envelope at a satisfactory rate. The invention is notlimited to the use of a series inductance, as various known methods ofimpedance matching may be used to increase the power efficiency.

Although the exact range of usable frequencies for the AC. source hasnot been determined, the preferred range is in radio frequencies fromabout 25,000 cycles per second upwardly, and successful operation hasbeen attained at a frequency of 50 megacycles. Neither the upper norlower frequency limits are known, although it is known that as thefrequency becomes lower, the rate of cleansing is reduced. A potentialof as little as 30-40 volts peak-to-peak is adequate for the purpose, inthe particular embodiment described.

In operation, after the envelope 11 is placed in position on the basestructure 12 as shown in FIG. 2, the mercury diffusion pump is startedto lower the pressure within the tube to about 13 microns of mercury.The gas discharge is started between the mercury pool 23 and theauxiliary anode 34 by means of a voltage pulse supplied to the igniter24 by momentarily moving the switch 55 from its solid line position toits dotted line position. Thus the capacitor 56 discharges through theigniter 24 and mercury pool 23, and a direct current discharge ofapproximately 3 amps. at 15 volts potential drop is established betweenthe mercury pool 23 and the auxiliary anode 34. The main anode 36 isconnected through its lead 37 and variable resistor 58 to the ZOO-voltpower supply, and the main discharge of approximately 5 amps. at 30volts potential drop is established between the mercury pool 23 and themain anode. This causes the anode section of the tube, that is, thespace inside the envelope 11 above the grid 32, to become filled with alow-pressure mercury plasma with a density of the order of 10 -10 ionsper cubic centimeter. The target 40 attached to the target electrode 38becomes surrounded by this highdensity plasma, and, because the targetis slightly negatlve with respect to the main anode, it is bombarded bymercury ions. Part of the material sputtered from the target by theionic bombardment is deposited on the substrate, while part tends to bedeposited on the inside of the envelope 11. However, because of theelectrode 50 and the potential impressed thereon, the envelope itselfadjacent the electrode is subjected to ionic bombardment and anymaterial from the target deposited on the envelope in that area issputtered off.

Sputtering, and hence cleansing of the envelope occurs, if the materialof the envelope is an insulator, because of the alternating potentialimpressed between the electrode 50 and one of the electrodes in thetube, which, in the present case, is the main anode 36. If a directpotential is used instead of an alternating potential, a positive chargebuilds up on the envelope surface, which reduces the potential dropbetween the surface and the plasma to such an extent that there is noappreciable bombardment. With the alternating potential, however, theenvelope is not only bombarded by positive ions, but, during a positiveportion of each cycle, it is bombarded by electrons, which prevents apositive charge from building up on the material. Thus, the envelope issputter-cleansed for a major portion of each cycle of the alternatingpotential. The alternating potential may be approximately the same asthe voltage drop between the anode and cathode of the tube or, in thiscase, about 30 volts. It is pointed out that it may not be necessary toapply the alternating potential continuously, and intermittent cleansingof the envelope may be sufiicient.

Although particular values of voltage and current have been mentioned,it is to be understood that these are illustrative only and theinvention is in no way limited to any specific operating conditions.

FIG. 5 shows a modified circuit diagram wherein an alternating potentialis placed on the target electrode as well as on the external electrode.Otherwise, the operation of the apparatus is the same as that previouslydescribed. It will be apparent, however, that by using an alternatingpotential on the target electrode, a nonconducting target may besputtered in the same fashion as the inner surface of the glass envelopemay be sputtercleansed.

FIG. 6 illustrates the teachings of the invention applied to removing adeposit from a fluorescent lamp. A conventional fluorescent lamp 70,having an electrode 71 and electrical connector 72, also has a deposit73 inside its envelope caused by material sputtered from the electrode71 during operation. To remove the deposit, an electrode 74 is placedadjacent the deposit, outside the envelope, and a source of alternatingpotential (not shown) is connected between the two electrodes 71 and 74.If the lamp is in operation to provide the plasma for the ionicbombardment of the deposit, only a small alternating field is requiredto remove the deposit. If the lamp is not in operation, the plasma maybe produced by the applied alternating field itself, in which case ahigher potential is required to produce the necessary plasma. It hasbeen found that such a deposit may be removed in a matter of seconds. Itis also pointed out that the alternating electric field may be appliedbetween two electrodes on opposite sides of the lamp with the deposittherebeween.

Although several embodiments and modifications have been described, itis apparent that many more modifications may be made by one skilled inthe art without departing from the scope and spirit of the invention.

We claim:

1. In apparatus having a transparent envelope and electrode means forproviding an electric gaseous discharge within the envelope wherebydeposits tend to coat a given area of the inside surface of theenvelope, a transparent electrode adjacent the outer surface of theenvelope and approximately opposite to said given area coated with adeposit to be removed, and a source of radio frequency alternatingpotential connected between said transparent electrode and saidelectrode means to provide an alternating radio frequency electric fieldtherebetween.

2. In an apparatus having a transparent envelope and electrode means forproviding an electric gaseous discharge within said envelope, saidgaseous discharge forming deposits covering an area of the insidesurface of said envelope, the improvement for cleaning said deposits offsaid area of said inner surface which comprises:

a transparent electrode adjacent the outer surface of said envelope andopposite to said area of said inner surface covered with a deposit to beremoved, and

a source of radio frequency alternating potential connected across saidtransparent electrode and said electrode means for establishing a radiofrequency field in said envelope adjacent to said covered area to removesaid deposit and clean said area.

3. The method of cleaning an apparatus including a nonconductingenvelope containing an ionizable gas, said apparatus also includingelectrode means for establishing a gas discharge, said gas dischargeincluding ions of said gas within said envelope, said gas dischargebeing effective to coat a selected area of the inside surface of saidenvelope with a deposit, which method comprises:

energizing said electrode means to provide said gas discharge in saidenvelope, and

establishing a radio frequency alternating field across said selectedarea to develop a radio frequency potential on said selected area andbombard said selected area with said ions so that said deposit isremoved from said selected area.

4. A method of cleaning a deposit from a given area of the insidesurface of an enclosure provided with electrode means for establishingan electric gaseous discharge therein, which comprises:

energizing said electrode means to provide said electric gaseousdischarge in said enclosure, said discharge including ions;

positioning an electrode on the outside surface of said enclosureopposite to said given area of said inside surface of said enclosure,and

applying a radio frequency alternating potential across said electrodemeans and said electrode for bombarding said given area of said insidesurface with said ions to clean said deposit from said inside surface.

References Cited by the Examiner UNITED STATES PATENTS 2,774,013 12/1956Willoughby 313-20l X 2,877,338 3/1959 Berge 204-192 X 2,947,913 8/1960Trostler 313-201 X JOHN W. HUCKERT, Primary Examiner.

BENNETT G. MILLER, DAVID J. GALVIN, JOHN W. HUCKERT, Examiners.

1. IN APPARATUS HAVING A TRANSPARENT ENVELOPE AND ELECTRODE MEANS FOR PROVIDING AN ELECTRIC GASEOUS DISCHARGE WITHIN THE ENVELOPE WHEREBY DEPOSITS TEND TO COAT A GIVEN AREA OF THE INSIDE SURFACE OF THE ENVELOPE, A TRANSPARENT ELECTRODE ADJACENT THE OUTER SURFACE OF THE ENVELOPE AND APPROXIMATELY OPPOSITE TO SAID GIVEN AREA COATED WITH A DEPOSIT TO BE REMOVED, AND A SOURCE OF RADIO FREQUENCY ALTERNATING POTENTIAL CONNECTED BETWEEN SAID TRANSPARENT ELECTRODE AND SAID ELECTRODE MEANS TO PROVIDE AN ALTERNATING RADIO FREQUENCY ELECTRIC FIELD THEREBETWEEN.
 3. THE METHOD OF CLEANING AN APPARATUS INCLUDING A NONCONDUCTING ENVELOPE CONTAINING AN IONIZABLE GAS, SAID APPARATUS ALSO INCLUDING ELECTRODE MEANS FOR ESTABLISHING A GAS DISCHARGE, SAID GAS DISCHARGE INCLUDING IONS OF SAID GAS WITHIN SAID ENVELOPE, SAID GAS DISCHARGE BEING EFFECTIVE TO COAT A SELECTED AREA OF THE INSIDE SURFACE OF SAID ENVELOPE WITH A DEPOSIT, WHICH METHOD COMPRISES: ENERGIZING SAID ELECTRODE MEANS TO PROVIDE SAID GAS DISCHARGE IN SAID ENVELOPE, AND ESTABLISHING A RADIO FREQUENCY ALTERNATING FIELD ACROSS SAID SELECTED AREA TO DEVELOP A RADIO FREQUENCY POTENTIAL ON SAID SELECTED AREA AND BOMBARD SAID SELECTED AREA WITH SAID IONS SO THAT SAID DEPOSIT IS REMOVED FROM SAID SELECTED AREA. 